This invention relates to a method and apparatus for carrying out electrical resistance seam welding with the use of a continuous wire electrode which passes successively over a first electrode reel and then over a second electrode reel, confronting the first reel, via a plurality of guide and deflection rollers, the wire electrode portions travelling over the reels being pressed at a predetermined welding pressure by the reels being pressed together so that overlapping ends of a workpiece disposed between the wire portions may be welded together while an electric current is applied so as to pass from one of the reels through the wire electrode and workpiece ends to the other reel. The wire electrode is motordriven for forwardly advancing the workpiece ends.
A welding apparatus of this general type is disclosed in Swiss Pat. No. 436,513. During operation of this known apparatus, the cross-section of the electrode wire is reduced during its travel over the first electrode reel while under the influence of a welding force and of the temperature prevailing at the welding area, while at the same time the initial length of the electrode wire is caused to increase. In order to prevent the elongated portion of the electrode wire from speeding up while passing over the second electrode reel, as compared to the speed of travel of the unelongated portion of the wire travelling over the first electrode reel, means are provided for forming and receiving a wire loop between the two reels, as well as for taking up this loop when there is no pressure applied to the reels.
Another known apparatus for carrying out electric resistance seam welding is disclosed in Swiss Pat. No. 519,961 in which a wire electrode having an initially round cross-section is pressed into a flattened cross-sectional shape prior to its travel over the first electrode reel by means of a driven pair of press rollers. During this rolling process, the wire electrode assumes a higher strength and a correspondingly smaller elongation so that there is no further elongation of the wire at the welding area, thus eliminating the need for a loop-takeup device. Hence, the wire electrode used in this apparatus is a "hard wire".
This concept relating to use of such a "hard wire" is discussed in more detail in Swiss Pat. No. 536,163 which describes a round wire, for example a copper wire, as being finish-rolled unitl it reaches a 50% higher tensile strength at the elastic limit, for example 13 kg-f/mm.sup.2, and preferably a 100% higher tensile strength at the elastic limit, for example 18 kg-g/mm.sup.2, than prior to the finish-rolling operation.
In principle, when using a hard wire as a wire electrode for electrical resistance seam welding, elongation of the electrode wire during its first travel through the welding area can be substantially avoided. However, the hard wire does not conform well to the workpieces to be welded under the influence of the welding pressure, relatively large deflection forces are required for appropriately passing the hard wire over the electrode reels and deflection rollers, and the hard wire is more brittle which increases the danger of operational breakdowns.